1. Field of the Invention
The present invention relates to an ink, particularly an ink for use in ink jet recording, and an ink jet recording method utilizing the same. The present invention also relates to an ink capable of improving (namely extending) a maximum elapsed time from the end of ink ejection by an ink jet ejection unit to a time when a next normal ink ejection is still possible (such time being called first-ejection time in the present specification), while maintaining a sufficient density in a recorded image, and relates to the equipment utilizing such ink. The ink of the present invention is most suited for an ink jet head of an ejection amount of 10 picoliters or less, and is applicable to all equipment capable of recording on various recording media (printing media) such as paper, cloth, leather, non-woven cloth, OHP sheets etc., for example office equipment such as a printer, a copying apparatus, or a facsimile employing an ink jet recording method.
2. Related Background Art
An ink jet printing method has various advantages such as a low noise level, a low running cost, the possibility of high-speed printing, easy compactization of the apparatus, easy color image formation etc., and is therefore widely employed in a printer, a copying apparatus etc. In such a printer or the like, ink to be employed is selected in consideration of printing characteristics, such as an ejection property and a fixing property, and a print quality, such as bleeding in the printed image, optical reflective density, and color developing property.
It is well known that the ink is divided, in terms of a colorant contained therein, into two categories: a dye based ink and a pigment based ink. Among these, the pigment ink containing a water-insoluble colorant has advantages such as superiority in water resistance and light fastness, and sharp character quality in comparison with the dye ink containing a water-soluble colorant.
As an example of pigment ink to be employed in the ink jet recording, a pigment ink utilizing a dispersant is known. For example, Japanese Patent Application Laid-open No. 5-179183 (JPA 5-179183) discloses a pigment ink utilizing, as a pigment dispersant, a block polymer of the so-called AB, or BAB type. Also Japanese Patent Application Laid-open No. 7-53841 (JPA 7-53841) discloses a pigment ink utilizing a triblock polymer of the ABC type as a dispersant. For improving the fixing speed of a pigment dispersed by a dispersant of such type and increasing the optical density and the water resistance thereof, JPA 7-53841 is known to disclose, as essential conditions, two solvents constituted of a polyol/alkylene oxide condensate and a cyclic amide derivative such as 2-pyrrolidone for dissolving the same. JPA 7-53841 recognizes that a drying fixation speed of the ink and a solvent evaporation from the nozzle or a clogging therein are mutually contradictory factors, and, discloses an invention for improving the fixation speed of the pigment utilizing a dispersant, in comparison with a comparative example employing a cyclic amide derivative alone such as 2-pyrrolidone. JPA 7-53841 does not describe at all an ink jet head to be used.
Also there is known a pigment ink employing a self-dispersible pigment, without utilizing a dispersant such as the aforementioned block polymer. For example, WO 96/18695 and WO 96/18696 disclose a pigment utilizing carbon black on which a hydrophilic group is directly bonded to the surface. Such pigment ink aggregates without penetrating into a printing medium (recording medium) in an image dot formation. As a result, the colorant (pigment) becomes unevenly distributed in the vicinity of the surface of the printing medium.
An image formed by a pigment ink, observed in a single dot, has a uniform density distribution, and is considered acceptable in its outer shape. However, in the ink jet recording, the dot diameter of the pigment ink does not spread much. Therefore, a further improvement in the pigment is desirable in order to obtain a larger area factor in the ink jet recording. Also in the conventional pigment ink, in case a printing medium on which such pigment ink is applied has an insufficient ink absorbing property, the pigment aggregates on the surface of such a medium, thereby resulting in an image deficient in the uniformity of the colorant fixation. Also not only does a pigment ink in which the aforementioned aggregation reaction is relatively strong cause an uneven aggregation as in the conventional pigment ink, but also it may generate a colorant-lacking portion of a “crack” shape in the pigment fixed on the printing medium. Such a “crack” has a relatively large size that is noticeable also by the bare eyes, so that such a “crack” itself deteriorates the print quality. Also such crack portion exposes the background of the printing medium, thereby resulting in a decrease in the overall optical density. Such a “crack” often appears on a printing medium bearing a coated layer (resin layer) for accepting the ink, such as a transparency film. This is because the aggregation of the pigment is influenced by a substance contained in the resin layer. Particularly, in the case where the resin layer contains a cationic substance, an ink of an anionic pigment shows a rapid aggregation. Basically, the aforementioned aggregation-related issues appear conspicuously in the case where a pigment ink alone is used for printing, but are found to occur also in an ink utilizing a dye and a pigment as colorants and requiring a dispersant for such pigment, as disclosed in Japanese Patent Application Laid-open No. 2-276873 (JPA 2-276873). For resolving such a “crack” problem, Japanese Patent Application Laid-open No. 11-240145 (JPA 11-240145) discloses an invention based on a combination of a self-dispersible pigment and a dye. Also U.S. Laid-Open No. 2003/0024434 Laid-open (US 2003/0024434) discloses an ink utilizing a combination of a special self-dispersible pigment, a dye and plural solvents (a humectant and a penetrant) thereby satisfying high-speed fixation and optical density. US 2003/0024434 discloses, as an only example utilizing a printer of a trade name Lexmark Z51 (black ejection amount of 27 picoliters), a combination of self-dispersible pigment:dye=1:1 and 7.5 mass % of 2-pyrrolidone. US 2003/0024434 suggests a range of pigment:dye from 0.75:1 to 2.5:1, but such disclosure is limited to a range where the optical density is considered satisfactory. Also it discloses a permissible range of 2-pyrrolidone of 3 to 10 mass %, but no basis therefor is disclosed.
For the ink to be used in the ink jet recording method, it is important, in addition to the properties relating to the image quality, to secure stable ink ejection even in the case of intermittent ejections. In case an ink jet printing head having a plurality of nozzles is paused and is exposed to the air, there results evaporation of water or a solvent in the vicinity of the ejection orifice, depending upon a non-ejection time including a difference in the ejection history of each nozzle, whereby a normal printing operation cannot be executed in certain nozzles. Thus the ink is not ejected uniformly from the nozzles, thus generating a defect on the image or causing a deflection in the ejecting direction, thereby deteriorating the print quality. For this reason, in the conventional ink jet head, there is executed an ejection not for recording (such ejection being called a preliminary ejection) even by interrupting a recording operation, in order to apply a safety factor on a non-ejection time inducing such ejection failure, thereby securing the reliability of the head. Since such preliminary ejection often interrupts the recording operation, it is proposed, in order to shorten the time of suspension as far as possible, to provide ink receiving portions on both ends outside a recording area and to execute the preliminary ejection in a closer ink receiving portion depending on the position of the scanning head (for example Japanese Patent Application Laid-open No. 8-118674 (JPA 8-118674), or to execute the preliminary ejection on an end portion of a paper constituting the recording medium (for example Japanese Patent Application Laid-open No. 7-314708 (JPA 7-314708).
It is generally considered that an ink employing a dye as the colorant does not influence a next printing even in case of exposure to the air for several seconds because the dye is water-soluble, while an ink employing a pigment as the colorant, because the pigment is insoluble in the liquid medium of the ink, tends to cause a clogging of the ejection orifice of the printing head in case of a prolonged exposure and shows a higher possibility of influencing a next printing operation, in comparison with the dye ink.